This invention relates to improvements in a flowmeter of a hot wire type in which the fluid flow rate is determined by the measurement of heat transferred from an electrically heated wire placed in the fluid flow to the fluid.
In the accompanying drawings, FIGS. 1 and 2 show the conventional flowmeter of this type in which a hot wire is placed on a plane normal to the direction of the flow. In FIGS. 1 and 2, a wire 3 in a I shape or an inverted V shape is placed on a plane normal to the flow in a fluid passage 2 of a pipe 1 and both ends of the wire 3 are connected to terminals 4a and 4b, respectively. Reference numeral 5 is a means for supplying an electric current to the wire 3 and measuring electrically the temperature drop of the wire.
When the thus arranged wire 3 is heated by passing an electric current through it and exposed to the fluid flow, the hot wire 3 is cooled by the flow and consequently the electric resistance of the wire 3 decreases. Since the heat release from the wire 3 to the fluid, which is known from the temperature drop of the wire 3, is proportional to the flow rate, the flow rate can be determined by the measurement of the resistance change of the wire or the voltage change between the terminals.
Such a conventional flowmeter of a hot wire type is useful for a fluid flow in a steady state, but not for a flow in which either the fluid velocity varies along the direction of the flow, as shown in FIG. 3 of the accompanying drawings, for example, an air flow in an intake manifold of an automobile engine equipped with an electronic fuel injection control system, or the fluid velocity varies with time as a pulsating flow as shown in FIG. 4 of the accompanying drawings. When a flow has such a spatial and/or temporal distribution of the velocity, the actual value of the flow rate can not be obtained because the heat release from the hot wire placed on a plane normal to the flow does not always correspond to the mean value of the velocities at various cross sections of the fluid passage, but corresponds only to the velocity at one cross section.